He said it was a "worrying" sign rather than a cause to panic but said employers would have to think about increasing their own contributions to funds to get levels back up again.

Mr Wilson added that factors which had been predicted, such as schemes getting more expensive due to people living longer and lower interest rates, were beginning to show through in the results.

However, not all firms were reporting a gap between the value of assets and benefits members had earned, and 83 of the FTSE 100 companies claimed they had assets greater than the level needed to meet benefits, including 20 firms which said they had funding levels which exceeded this level by 25%.

But Mr Wilson warned that a change in the way companies assess the value of their scheme, forcing them to use up-to-date market valuations, was likely to lead to further increases in the number of under-funded schemes reported in the next few years.

He said: "It's likely that future years' disclosures will paint a more negative picture of pension scheme funding than we've seen this year."

Bacon & Woodrow also found that the cost of running pension schemes had reduced slightly since the previous year.

The highest reported cost of a scheme was 14.7% of the company's total salary costs, down from 19.5% last year, and the average cost was 4.1% compared with 5%.